a) Field of the Invention
The present invention relates to carbon-fiber-reinforced polyimide resin compositions having excellent mechanical strength.
b) Description of the Related Art
Thermoplastic polyimide resins led by "Ultem" (trade mark; product of General Electric Company) are superior in heat resistance and mechanical strength to general-purpose engineering plastics and are hence called "super engineering plastics". Their application is now widely being investigated for electric and electronic equipment and appliances, machinery, cars and the like.
With the recent advancement of technology, there is an increasing demand for novel thermoplastic polyimide resins having heat resistance and mechanical characteristics comparable with or better than "Ultem".
For example, U.S. Pat. No. 4,847,349 discloses a process for producing a polyimide resin by reacting an ether diamine with a tetracarboxylic dianhydride. Further, the production of a polyimide resin by the reaction of 3,3'-diaminobenzophenone and a tetracarboxylic dianhydride is also disclosed in Japanese Patent Application No. 167392/1989, etc. These related pieces of art have both furnished novel polyimide resins which have heat resistance and mechanical characteristics not available in the past.
With a view to further improving characteristics of these polyimide resins, especially their mechanical strength, a fibrous reinforcing material, typically carbon fibers is generally incorporated. However, an epoxy resin is usually employed as a binder for carbon fibers because carbon fibers are often used in carbon-fiber-reinforced plastics formed of an epoxy resin as a matrix. The epoxy resin binder is therefore effective where a thermosetting resin such as an epoxy resin is used as a matrix. The epoxy resin binder however has poor adhesion to polyimide resins so that the epoxy resin binder cannot provide resin compositions having good mechanical strength. It may be contemplated to use a polyamide resin as a binder for carbon fibers as disclosed in Japanese Patent Application Laid-Open No. 106752/1978. A high temperature of at least 300.degree. C. is generally required to mold or otherwise form a polyimide resin. The binder may hence undergo thermal decomposition during the molding, thereby causing problems such as the formation of voids and strength reduction at welded portions.
Further, as disclosed in Japanese Patent Application Laid-Open No. 120730/1981, it may also be contemplated to use carbon fibers bound with an aromatic polysulfone resin. However, this method has still not fully satisfied the required characteristics.